the copper-aluminum equilibrium phase diagram is a good illustration of why a lot of alloys are in the “90% A, 5% B, <1% each of C, D, E, etc” category
once you get into that middle area, not only do does your equilibrium phase composition get more intricate, but it also gets way hard to predict the true microstructure you will get (phase diagrams do not capture kinetics)
obviously, computational approaches have gotten leagues better, but you still have to get your model and your process to actually match
so happy to see metallurgy getting love on my dash :) it's also important to note that in the middle, you're much more likely to get very brittle intermetallic phases that typically don't give the best resultant properties regardless. this is also part of why alloys are often dominated by one element, to produce a microstructure that is mostly a ductile metal matrix with hard intermetallic precipitates for strengthening
